300SD Trans. Slipping

I have developed a problem with the transmission on my 1982 300 SD. Whenever the car is in first gear, it slips, meaning it momentarily feels as though it goes into nuetral with no acceleration.
Two weeks ago the car was not shifting at all, so I changed the trans fluid and the filter, and then it started to work great.
Last week, however, this started. Once it gets into 2nd gear, it works fine. It is VERY annoying because I cannot accelerate from a stop sign or a traffic light. The 2-3 Shift also feels softer than before. Can this be the beginning of the B2 Problem, or can this be caused by some mis-adjustment. The vacuum lines all tested to be good.
Any help is appreciated.
Thank You
Tony Konstantinidis